Investigation of the role of fishbone activity in the formation of internal transport barrier in HL-2A plasma

Abstract

A tokamak scenario with q(0) close to 1 has been achieved on HL-2A tokamak, which is an internal transport barrier (ITB) at low central shear or a steady-state ITB combined with an H-mode edge barrier. In this scenario, the formation of an ITB with a steep ion temperature profile is observed to be closely linked to the q = 1 magnetic surface and magnetohydrodynamic (MHD) activities around it, such as long-lived mode (LLM) or fishbone activities. Experimental evidence and simulation analysis suggest that the fishbone activities can induce a poloidal flow, which is beneficial for the suppression of turbulence in the plasma core region. Furthermore, an application of central electron cyclotron resonance heating (ECRH) to such beam-heated weak shear ITB discharges leads to a substantial effect on central MHD stability, converting the LLM into fishbone activity and hence enhancing the strength of the ITB with a much stronger gradient. Moreover, ITBs in combination with an H-mode barrier were achieved for 10 confinement times with βN = 2.0.